The invention relates to waveguide arrays such as fiber optic face plates, and other fiber optic imaging devices. In particular, the invention relates to such devices having integral contrast enhancement and a method for producing the same.
Fiber optic face plates (FOFPs) are exemplary of image transfer devices generally consisting of coherent lattice arrays of step index waveguides which act as image plane transfer devices, i.e. they "pipe" an image from the input surface to the output surface. Other known image transfer devices include image conduits, microconduits, tapers, inverters, flexible image scopes, light guides, individual fibers and fused fiber optics.
A fragmentary cross-section of a known FOFP 10 is illustrated in FIG. 1. The structure of the FOFP 10 generally consists of a two phase array of optical waveguides each including a high refractive index (e.g., n.sub.1) core glass 11, each of which is surrounded by a contiguous second phase of lower index (e.g., n.sub.2) cladding glass 12.
The glasses most commonly used for FOFPs are high refractive index lead silicates for the core material (e.g. Schott F7 or SF6) and low refractive index borosilicates for the cladding (e.g. Corning 7052). The core glass 11 has a center axis 13. Immediately adjacent to the cladding 12 on either side are neighboring cores 11. A typical FOFP used in imaging applications utilizes waveguides packed in a hexagonal lattice with a center-to-center spacing of 6 .mu.m and a core diameter of 5 .mu.m. The faces of the exemplary FOFP consist of about 70% core and about 30% cladding area. As sometimes used herein, the term open area ratio is the core area divided by the total active area of the device.
The mode of light transmission in imaging devices like the FOFP 10 in FIG. 1 is as follows. An incident light ray 14 at some incident angle .THETA..sub.i relative to the center axis 13 enters the core 11 of the FOFP. If the sum (90-.THETA..sub.i) is greater than the critical angle for total internal reflection .THETA..sub.c, the incident ray 14 is not refracted into the cladding 12 but is instead completely reflected down the core 11, as shown. Incident rays 14' where the sum (90-.THETA..sub.i) is less than .THETA..sub.c are refracted and pass into the cladding phase 12, where they are free to enter adjacent cores. This is the most commonly discussed mode of cross-talk between constituent fiber elements in FOFPs.
A commonly used measure of the acceptance angle of a waveguide, within which total internal refraction occurs, is the Numerical Aperture (N.A.), defined as N.A.=sin(90-.THETA..sub.c)=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.0.5. Thus for the case where the N.A. of each waveguide in the FOFP is 1.0, .THETA..sub.c is 0.degree., and light up to 90.degree. off normal incidence is totally internally reflected. For the case of an N.A. of 0.5, .THETA..sub.c is 60.degree., and incident light of up to 30.degree. off normal incidence is totally internally reflected.
Also shown in FIG. 1 is the case where an incident ray 15 of angle .THETA..sub.i enters the cladding phase 12 instead of the core. In this case, instead of being totally internally reflected, the ray 15 is partially refracted (shown as a dotted line) into the core 11. The other portion of the incident ray 15 (shown as a solid line) is reflected back into the cladding 12, which being a common phase leads to diffusion over a wide area. Portions of reflected rays also enter adjacent cores at each subsequent reflection, leading to cross-talk. This type of cross-talk is not commonly considered as a source of image quality degradation in FOFPs but it is quite significant.
Absorbing glasses are often incorporated into FOFPs to suppress cross-talk. These materials, generally termed Extra-Mural Absorbers (EMAs) are incorporated in three ways, namely: a) as absorptive coatings applied to the outside of each individual waveguide (circumferential EMA); b) a fraction of the waveguides are randomly substituted by absorbing fibers (substitutional EMA); and c) absorbing fibers are inserted into the interstitial packing vacancies in the array (interstitial EMA).
At first glance, circumferential EMA would seem to be the most effective option. In practice it is the least effective as the absorbing glasses commonly used have insufficient absorption intensity at their final thickness (&lt;1 .mu.m) to be effective. Substitutional EMA is also ineffective and also acts as an obscuration or defect. The most effective configuration to date is interstitial EMA. However, interstitial EMAs are usually wavelengths selected and do not exhibit broad band effects. Concepts for incorporation of various types of EMAs may be found in U.S. Pat. Nos. 3,060,789, 3,247,756, 3,253,500, 3,387,959, 4,011,007 and 3,836,809.
Whether effective to satisfactorily prevent cross-talk or not, EMAs, by definition, are absorbers. Accordingly, the image intensity is attenuated and hence, image quality degrades as a result of absorption as the device thickness or length increases to any significant degree. Typically a device longer than a few millimeters will have unsatisfactory image quality for certain applications. The prior art thus requires a trade off between contrast and intensity.
From the above discussion it is apparent that it would be highly desirable to suppress cross-talk by preventing incident light from entering the cladding. In principle this could be done by laying down an opaque mask having holes of diameter and spacing equal to those of the cores of the elements of the FOFP. In practice this is impossible because of the high degree of misorientation and packing imperfections found in FOFPs.
The concept of darkening glass surfaces in a reducing atmosphere is known. In U.S. Pat. No. 2,314,804, glasses containing PbO or CuO are exposed to Hydrogen at elevated temperatures to create opaque surface layers which are later selectively removed by grinding and polishing to create decorative effects.
In U.S. Pat. No. 2,339,928, lead-containing fibers are exposed to hydrogen at elevated temperatures to achieve a fast color. Treatment temperature is limited to no more than 400.degree. C.
In U.S. Pat. No. 3,650,598 high temperature reduction in Hydrogen for gradient index (GRIN) cylindrical rod optics to darken the outer walls of the cylinder and suppress cross-talk is shown. This patent is specifically restricted to GRIN optics and does not teach incorporation into an FOFP.
In U.S. Pat. No. 4,989,960, a process for blackening the perimeter of lenses in a reducing atmosphere at elevated temperature to suppress reflected stray light is shown.
The above examples either teach the general concepts of hydrogen reduction to for blackened surface layers or teach the use of such blackened layers along the outer length of a cylinder (either rod or lens) to suppress reflected stray light by absorption.
U.S. Pat. No. 3,582,297 teaches the concept of generating an opaque surface mask to prevent stray light from entering the cladding. The patent teaches the use of high temperature ion exchange of silver with constituents of the cladding glass (i.e. mobile alkali ions). The silver enriched layer is preferably reduced by hydrogen at an elevated temperature to form a relatively deep opaque surface layer (e.g. &gt;15 .mu.m) which then acts as a mask to prevent stray light from entering the cladding.
The resulting product and method taught by U.S. Pat. No. 3,582,297 has several significant shortcomings. Generally only a limited amount of silver may be ion-exchanged. This results in less intense absorption, requiring relatively thick surface layers to be effective. Silver ion exchange generally uses a molten salt bath or paste of silver salts. Such solutions often cause corrosion or deterioration of the polished surface layer (especially on the core glass of the composite), which degrades the optical performance of the device. Such corrosion is extremely undesirable in demanding applications such as windows for Charge Coupled Devices (CCDs), where extremely low light scattering and obscuration are desired. Silver ion exchange results in an increase in fluorescence on exposure to short wavelength radiation (ref. W. Weyl et al., "On the fluorescence of atomic silver in glasses and crystals", J. Electrochem. Soc., vol. 95, p. 70, 1949). Such an effect is highly undesirable in many low light imaging systems (e.g. image-intensified CCDs) where the fluorescence would significantly degrade image contrast. Silver ion exchange followed by reduction produces surfaces which are electrically conductive. This conductivity has been deliberately exploited in the prior art to produce conductive circuits on glass surfaces. Surface conductivity is highly undesirable for applications such as image-intensified CCD systems where the fiberoptic must act as an insulator.